1. Field of the Invention
The present invention relates, in general, to a method for making a tungsten plug of a highly integrated semiconductor device, and more particularly to improvements in easily controlling etch back processes for blanket tungsten and in key hole in the contact hole along with the method.
2. Description of the Prior Art
When carrying out multilayer wiring process for a highly integrated memory device or logic device, there is generally utilized a plug such that a metal is buried in a contact hole in order to vertically connect a metal wire with a lower conductive layer via the contact hole.
Hereinafter, description for a conventional method for making a tungsten plug will be given along with the problems generated therefrom for better understanding of the background of the invention.
For the purpose of allowing an upper tungsten to come into contact with lower silicon substrate, initially, the predetermined portion of an insulating film which is formed on the silicon substrate is subjected to an etch process to form a contact hole and then, a conjunction layer is formed on the insulating film by sputtering or chemical vapor deposition to improve the adhesiveness of tungsten thin film. Subsequently, blanket tungsten is deposited on the resulting structure, followed by the application of etching process to a predetermined portion of the blanket tungsten and to the conjunction layer. As a result of performing these steps, a tungsten plug is formed.
However, the conjunction layer is deposited in a inhomogeneous thickness in the contact hole. In other words, the conjunction layer is deposited in a large quantity, e.g. thickly, on the upper portion of the contact hole whereas it is deposited in a small quantity, e.g. thinly, on the lower portion of the contact hole. As a result, the upper portion of contact becomes narrower than the lower portion. Therefore, even if blanket tungsten is deposited, there occurs a key hole in the contact hole, which, in turn, complicates etch back processes for the blanket tungsten. For example, it is difficult to control etching process for the blanket tungsten and the junction layer. In addition, the difficult control for etching results in an expansion of key hole at the plug formed in the contact hole. Consequently, the production yield becomes lowered due to the difficulty of control for process as well as device manufactured by the conventional method is deteriorated in reliability.